On 14 July 2005 Sweden proposed, within the framework of the Stockholm Convention on persistent organic pollutants (POPs), to add a new group of POPs (perfluoroctane sulfonates, PFOS) to those already covered by the Convention. Discussions had taken place within the Council and its Working Party on International Environmental Issues and with the Commission over the substances to be proposed by the EU and its Member States for listing under both the Stockholm Convention and the Aarhus Protocol to the Convention on Long Range Transboundary Air Pollution, but it had not at this stage been agreed at EU level to include PFOS among those to be proposed. The Commission took the view that this unilateral act by Sweden was a breach of its duty of loyal cooperation under Article 10 EC (now Article 4(3) TEU) and brought an action before the Court of Justice, which found that Sweden had indeed failed to fulfil its obligations under Article 10 EC. The case concerns the joint participation of the EU and its Member States in multilateral environmental agreements (MEAs) and carries implications for mixed agreements more generally. To what extent is Member State action within the structures established by an MEA constrained – as a matter of EU law – by the fact that it participates not only in its own right but also as a Member State of the EU? What does the duty of loyal cooperation require of the Member States participating under shared competence in a mixed agreement alongside the Union? If they are required to act only by way of common Union position does this not come perilously close to the position under exclusive Union competence? How is a ‘concerted Union action’ defined and how might it be identified, if a formal Council decision is not required? How much time should a Member State allow for the internal decision-making within the EU to bear fruit? Apart from these issues relating to the management of shared competence, this case also prompts some reflections on policy coherence within the Union: on the coherence between internal and external EU environmental policy; on coherence between environmental protection and other EU policy objectives, including development cooperation; and on the impact of Union policy choices on a Member State’s policy priorities.